Overhead in general term refer to the additional resources required to perform the a task.

For example, In computer system, allocation of resources, scheduling, security, processing, I/O control etc are referred as overhead.

Additional resources required by the operating system to read the data or process the data, where instructions are stored by operating system about the data like how to process the data , how to translate the data in to machine code, how to store the data etc. are known as a overhead for the data.

So overhead is nothing but additional space (resources) required or blocked by the operating system to work on that data.

In an accounting definition : overhead is nothing but additional expenses required to run your business like electricity, water supply etc which do not directly affect the sales of your product but it is an expense required to run your organization.